Slit radiography apparatus

ABSTRACT

A slit radiography apparatus is provided with an absorption device comprising electrically controllable piezoelectric tongues. Upper and/or lower deflections of the tongues are limited to a maximum by trips outside the X-ray fan beam. More than one strip can be present as seen in the length direction from the fixed ends of the tongues Thus deflections of the tongues are limited and great phase changes in the control signals without resultant oscillations can take place during the time the tongue are restricted in their movement by the strip.

This is a continuation of application Ser. No. 07/910,251, filed asPCT/EP91/00174 Jan. 28, 1991.

The invention relates to a slit radiography apparatus provided with anabsorption device which interacts with a slit diaphragm and whichcomprises electrically controllable piezoelectric tongues having a fixedend and a free end, and also a control device which feeds electricalcontrol signals to the tongues during operation.

Such an apparatus is known, for example, from Dutch Patent Application8400845. The known apparatus comprises an X-ray source which is capableof scanning a patient or object to be examined with a flat fan-shapedX-ray beam via a slit-type diaphragm. In order to obtain an equalisedX-ray image, an absorption device comprising a number of adjacentlysituated absorption elements which can be brought into the X-ray beam toa greater or lesser extent under the control of electrical controlsignals is provided near the slit diaphragm.

Each absorption element can affect a sector the fan-shaped X-ray beam.The control signals are obtained by means of a detector which isprovided behind the patient or the object and which measures the amountof radiation transmitted for each sector of the X-ray beam and providesa corresponding electrical control signal.

As absorption elements, use could be made of piezoelectric materialwhich is clamped at one end and whose other end can be swivelled intothe X-ray beam under the control of the abovementioned electricalcontrol signals. Such tongues already absorb X-ray radiation to acertain extent themselves, but they are usually also provided withspecial elements which absorb X-ray radiation at the free ends. In thelatter case, it is sufficient if the special elements which absorb X-rayradiation can be swivelled into the X-ray beam.

The tongues may be simple tongues of piezoelectric material which can bebrought into a curved state by means of an electrical control voltageapplied between the top and bottom face. The tongues may also beso-called bimorphous elements which are composed of two strips ofpiezoelectric material laid one on top of the other. The electricalcontrol voltage may then be applied between the interconnected outerfaces (top and bottom face) and the common central face.

The tongues can be controlled in such a manner that the position of thetongues with respect to the slit of the slit diaphragm is always matchedto the amount of radiation instantaneously transmitted through thepatient or the object in the associated sector of the X-ray beam, thatis to say, to the instantaneous, and therefore local, transmissivity ofthe patient for the object.

In the U.S. Pat. No. 4,916,713, the contents of which are regarded asincluded here by reference, a description is given of another method,according to which a so-called hardness modulation of the X-ray beam isused. According to the method known from U.S. Pat. No. 4,916,713, thefree ends of the tongues or the absorbing elements vibrate continuouslyat a predetermined frequency between a position completely or largelyclearing the slit diaphragm and a position completely or largelyblocking the slit. The amount of radiation instantaneously transmittedin a sector of the X-ray beam is controlled by varying the phase and/oramplitude of the vibration as a function of the instantaneoustransmissivity of the patient or the object in the sector concerned.

In all cases it is important that the tongues respond rapidly andaccurately to the electrical control signals. Thus, for example, it isimportant in the abovementioned U.S. Pat. No. 4,916,713 to be able tomatch the phase of the vibrating tongues rapidly to the instantaneouslocal transmissivity of the patient or of the object. Uncontrolledvibrations should also be prevented.

The object of the invention is therefore to provide an apparatus of thetype described above which meets the requirements mentioned. For thispurpose, according to the invention, such an apparatus is characterisedby at least one stop element which limits the maximum deflection of atleast a number of tongues.

The invention will be described in greater detail below with referenceto the attached drawing.

FIG. 1 shows, diagrammatically in side view, an example of a slitradiography apparatus provided with an absorption device havingpiezoelectric tongues;

FIG. 2 shows, diagrammatically in side view, a piezoelectric tongue inrelation to the slit of the slit diaphragm of a slit radiographyapparatus according to the invention;

FIGS. 3 to 6 inclusive show diagrammatically a number of exemplaryembodiments of stop devices for piezoelectric tongues of an apparatusaccording to the invention.

FIG. 1 shows, diagrammatically in side view, an example of a (known)slit radiography apparatus provided with an absorption device. Theapparatus shown comprises an X-ray source 1 which is capable of scanninga patient 4 or an object to be examined with a flat fan-shaped X-raybeam 3 in the direction, indicated by arrows 5, transverse to the planeof the fan-shaped beam 3 via a slit-type diaphragm 2. An X-ray detector6, which may comprise, for example an X-ray film cassette or aconcomitantly moving elongated X-ray image intensifier tube, picks upthe radiation transmitted through the patient and ensures the formationof the desired X-ray shadow image. The scanning in accordance with thearrows 5 takes place because, during operation, the X-ray source withthe diaphragm 2 swivels about an axis extending transversely to theplane of the drawing, preferably through the X-ray focus of the X-raysource, as indicated by an arrow 7.

To obtain an equalised radiogram, an absorption device 8 is used whichis capable of modulating the X-ray beam in each sector and which alsoswivels concomitantly with the X-ray source. The absorption devicecomprises adjacently situated piezoelectric tongues 9, also termedpiezoceramic tongues because the tongues are produced from ceramicmaterial having piezoelectric properties. Each tongue is able to affecta certain sector of the X-ray beam 3. For this purpose, the tongues aremounted in a fixed manner, for example clamped, at one end. By feedingsuitable electrical control signals to a tongue, it bends, with theresult that the free end penetrates the X-ray beam. The required controlsignals are provided by a detector 10 which is situated in this examplebetween the patient and the X-ray detector and which picks up theradiation transmitted through the patient at each instant and provideselectrical signals for each sector of the X-ray beam which are fed via aprocessing circuit 11 to the corresponding tongues. As shown, thedetector may be an elongated radiation detector moving synchronouslywith the scanning movement of the X-ray beam, but it may also be atwo-dimensional stationary detector.

FIG. 2 shows, diagrammatically in side view, a piezoelectric tongue 15which is suitably clamped in a fixed manner near one of the ends in asupport 16. The other end is free and is situated near the slit 17 ofthe slit diaphragm 2. The piezoelectric tongue is shown in the restposition in which the slit 17 is left completely free. This means thatthe X-ray beam 3 originating from the X-ray focus 18 of the X-ray source7 not shown in greater detail in FIG. 2 is able to pass the slit and thetongues freely.

The piezoelectric tongues are provided with diagrammatically shownconnecting terminals 19,20 for feeding electrical control signals.Moreover, in the example shown, the piezoelectric tongues are providedwith an absorption element 21 at the free ends. By means of a suitablecontrol voltage the result can be achieved that a piezoelectric tongueassumes a bent position such that the free end or the absorption elementmoves to a greater or lesser extent into the X-ray beam, as is indicatedby an arrow 22. As soon as the absorption element extends into the X-raybeam, the X-ray radiation is at least partially absorbed, and thereforeattenuated, at the position of the absorption element.

As already pointed out earlier, it is important that the tongues followthe control signals rapidly and accurately. The speed at which theposition of a tongue can be altered is related to the mass inertia orthe mechanical resonance frequency of the tongue. A large mass inertiaopposes rapid changes of position or vibration phase. Although supplyingstronger control signals may result in a more rapid response of thetongues, it also increases the risk of overshoot and of damage to thetongue. A tongue having a relatively low mechanical resonance frequency,that is to say, a resonance frequency which is in the same order ofmagnitude as the frequencies occurring in the control signals, may alsoenter into uncontrolled vibration and, for example, break as a result.

According to the invention, the problems outlined can be eliminated byusing a stop device which limits the maximum deflection of a tongue orof a tongue section during operation.

FIG. 3 shows diagrammatically a first example of such a stop device. Thestop device shown comprises two stop elements 31,32 which are providedon either side of the tongue 15 at a distance from the end 30 clamped ina support 16 and which limit the maximum deflection of the tongue. Assoon as the tongue collides with a stop element 31 or 32, said stopelement acts as a fixed point which temporarily takes over the functionof the support 16. The length of a tongue is consequently, as it were,reduced, as a result of which the resonance frequency becomes higher.The tongue consequently becomes more rapidly controllable.

The stop elements may have various forms and be produced, at leastinsofar as they lie in the region of the X-ray beam 3, from a materialwhich is transparent to X-ray radiation, for example plexiglass.

FIG. 4 shows an exemplary embodiment of stop elements 33,34 which eachcomprise a number of stop strips situated at a distance from each otheralong each of the tongues 15 in the form of ribs 35,36,37, or 38,39,40,respectively. As the deflection of a tongue 15 increases, it collidesfirst with the stop ribs 35,38, then with the stop ribs 36,39 andfinally, with the stop ribs 37,40. At the same time, the mechanicalresonance frequency of the tongue increases as it rests against morestop ribs.

The stop elements may alternatively be strips placed in step form whichform stop edges as shown in FIG. 5. The stop elements 40,41 shown inFIG. 5 each have three stop edges 42,43,44, or 45,46,47, respectively.In addition, the stop elements in this example also form the tonguesupport.

In the exemplary embodiments of FIGS. 3 to 5 inclusive, the stopelements form discrete stop ribs or edges. The stop elements may,however, also have, at least partially, a smooth shape, for example ashape according to a quadratic curve or a cubic curve or even accordingto a combination of different types of curves. A combination ofpartially smoothly shaped stop elements and one or more discrete stopedges is also possible.

FIG. 6 shows an example of stop elements 50,51 which, viewed from theclamped end of the tongues 15, first form a stop edge 52 or 53,respectively, and then have a smoothly shaped section 54 or 55,respectively. In combination with the smooth sections 54,55, discretestops 56,57 which are capable of interacting with a part of the tonguessituated near the free ends of the tongues have moreover also beenprovided in this example.

The rear edges 58,59 of the stop elements 54,55 may optionally come intocontact with the tongues as required during the operation of theapparatus and therefore function optionally as discrete stop edges.

In the examples shown, the stop elements are symmetrically arranged oneither side of the tongues. This is not, however, necessary. Thus, forexample, the embodiment of FIG. 3 could be modified in a manner suchthat the stop element 32 is situated further from the rest positionand/or further from the clamped end of the tongue than the stop element31.

In principle, the stop element at one side of the tongues could even becompletely omitted, with the result that the deflection of the tonguesis limited only on one side.

Since the properties of the individual tongues exhibit the greatestmutual differences at the maximum deflection of the tongues, the tonguesare preferably so arranged that the free ends of the tongues aresituated outside the X-ray beam at the maximum deflection. That is tosay, the free end of a tongue moves, at large deflection, behind theslit 17 of the fixed parts limiting the slit diaphragm and changedirection of movement at that point.

it is pointed out that, after the above, various modifications areobvious to the person skilled in the art. Thus, stop elements having asmooth shape as shown in FIG. 5 may also form the support, as shown inFIG. 4. Moreover, instead of three stop ribs or edges, more or less stopribs or edges may also be used, optionally in combination with one ormore smoothly shaped stop sections. These and similar modifications areconsidered to fall within the scope of the invention.

I claim:
 1. A slit radiography apparatus comprising an X-ray source (1),an X-ray detector (10) for sensing radiation passing through a body (4)to be radiographed, a slit diaphragm (2) positioned between said X-raysource (1) and said body (4) for forming a substantially planar X-raybeam (3), means for scanning (7) said body (4) with said planar X-raybeam (3) in a direction (5) transverse to a longitudinal direction of aslit (17) of said slit diaphragm (2), an absorption device comprising aplurality of electrically controllable piezoelectric tongues (9, 15)positioned along said longitudinal direction of said slit (17) forming acorresponding number of sections of said planar X-ray beam (3), each ofsaid tongues (9,15) having a fixed end and a free end, a control device(11) for feeding electrical control signals to said tongues (9, 15)having a fixed end and a free end, a control device (11) for feedingelectrical control signals to said tongues (9, 15) for continuousvibration thereof at a predetermined frequency characterized by at leastone stop element (31, 32; 35, . . . , 40; 40a, 41; 50, 51, 56, 57) forlimiting the maximum. deflections of at least a number of said tongues(9, 15), by said control signals being of such magnitude that saidtongues (9, 15) are limited in their deflections by said at least onestop element (31, 32; 35, . . . , 40; 40a, 41; 50, 51, 56, 57) for atleast part of the time of any vibration period and by means (11) forchanging phases of said electrical control signals during said part ofthe time of said vibration periods.
 2. Apparatus according to claim 1,characterized in that said at least one stop element comprises stopstrips (35, . . . , 40) situated one behind the other with a gap as seenfrom the fixed end of said tongues (9, 15), the distance between saidstop strips (35, . . . , 40) and a rest position of said tongues (9, 15)increasing as said stop strip (35, . . . , 40) is situated further fromthe fixed end of said tongues (9, 15).
 3. Apparatus according to claim2, characterized in that said stop strips are formed as mutuallyparallel ribs (35, . . . , 40) .
 4. Apparatus according to claim 2,characterized in that said stop strips are formed by edges of a saidstop element (40a, 41) in step form.
 5. Apparatus according to one ofthe preceding claims 1, 2, 3 or 4, characterized in that said at leastone stop element (31, 32; 35, . . . , 40; 40a, 41; 50, 51, 56, 57) hasat least one section (54, 55) which forms a smoothly varying stop pathaccording to a continuous curve.
 6. Apparatus according to one of thepreceding claims 1, 2, 3 or 4, characterized in that said at least onestop element (35, . . . , 40; 40a, 41; 50, 51, 56, 57) has at least onesection (54, 55) which forms a smoothly varying stop path according to acontinuous curve.
 7. Apparatus according to one of the preceding claims1, 2, 3 or 4, characterized in that said at least one stop element (31,32; 35, . . . , 40; 40a, 41; 50, 51, 56, 57) comprises a stop strip (56,57) which, during operation, interacts with an end section of saidtongues (9,1 5) situated near said free ends of the tongues (9, 15). 8.Apparatus according to one of the preceding claims 1, 2, 3 or 4,characterized in that said at least one stop element (31, 32; 35, . . ., 40; 40a, 41; 50, 51, 56, 57), said tongues (9, 15) and said slitdiaphragm 2 are so positioned that said free ends of said tongues (9,15) extend, at large deflections, outside a transmission region of saidslit diaphragm (2) defined by said slit (17) of said slit diaphragm (2).9. Apparatus according to one of the preceding claims 1, 2, 3 or 4,characterized in that said stop elements (35, . . . , 40; 40a, 41; 50,51, 56, 57) are placed on either side of the tongues (9, 15). 10.Apparatus according to claim 9, characterized in that said stop elements(31, 32) placed on either side of said tongues (9, 15) are mountedasymmetrically with respect to said tongues (9, 15).
 11. Method foroperating a slit radiography apparatus provided with an absorptiondevice (8) which interacts with a slit diaphragm (2) and which compriseselectrically controllable piezoelectric tongues (9, 15), each having afixed end and a free end and a control device (11) which feedselectrical control signals to said tongues (9,15) during operation, saidmethod comprising feeding control signals to said tongues (9, 15) forcontinuous vibration thereof at a predetermined frequency and changingphases of said control signals characterized in providing stop elements(31, 32; 35, . . . , 40; 40a, 41; 50, 51, 56, 57) for limiting maximumdeflections of said tongues (9, 15), controlling said control signals tobe of such magnitudes that said free ends are limited in theirdeflections by said stop elements (31, 32; 35, . . . , 40; 40a, 41; 50,51, 56, 57) and in changing phases of said control signals during thetime periods that said free ends are deflected against said stopelements (31, 32; 35, . . . , 40; 40a, 41; 50, 51, 56, 57).